Thermal spray coatings are formed by the impact and solidification of a stream of molten or semi-molten particles on a surface. The process combines particle acceleration, heating, melting, spreading and solidification in a single operation. Extensive use is made of thermal spraying in the aerospace, power generation and more recently in automotive industries to provide protective coatings on components that are exposed to heat, corrosion, and wear. Over the last decade, high velocity oxy-fuel process (HVOF) has been demonstrated to be one of the most efficient techniques to deposit high performance coatings at moderate cost. In this process, a mixture of fuel and oxygen ignites in a high pressure combustion chamber and the combustion products are accelerated through a converging-diverging nozzle such as that shown in FIG. 1. As a result, injected particles attain high velocity (above 400 m/s) at relatively low temperature (less than 2000° C.).
Referring again to FIG. 1, the HVOF gun is basically a converging-diverging nozzle to accelerate the gas flow to supersonic speeds at the gun exit. At the end of the gun, the flow is over expanded i.e. the Mach number is greater than one and gas pressure is lower than that of the ambient atmosphere. Because the flow is supersonic, the adjustment to the atmospheric pressure is through waves, oblique shocks or expansion waves. To reach ambient pressure the gases undergo a series of oblique shocks and expansion waves, which is called “shock diamonds”. Formation of the first shock diamond is shown in FIG. 2. This pattern will be repeated till the gas pressure reaches to the ambient pressure. In a typical HVOF process, seven to nine shock diamonds form in the ambient air.
A major technological advance achieved with the HVOF gun and process is to generate supersonic flows by which particles can reach high velocities. The reason is that for highly compressible flows the relative velocity between gas and particle can be greater than the local speed of sound. In this case, the compression shocks forming in front of the particles can accelerate particle to higher velocities (wave drag effect). This happens inside the gun where almost a uniform flow exists at each cross sectional area of the gun. Outside the gun, characteristic of the external flow becomes totally different from that of the internal flow, because of presence of a series of shock diamonds outside the gun.
Coating particles gain kinetic and thermal energy form the gas flow. Therefore, particle conditions (e.g. particle velocity, temperature, and trajectory) are a strong function of gas flow behaviour. Particles continuously accelerate inside the gun, whereas outside the gun they face several shocks and expansion waves. As a result, particles repeatedly (up to ten times) are accelerated and decelerated while passing through the external flow. Particles also deviate from their trajectory (which is along the nozzle centreline) because of the oblique shocks. The combination of these two effects causes some particles to not reach the critical velocities required for sticking to the substrate and become dispersed outside the gun. Consequently, the coating deposition efficiency and quality will be decreased. In practice, on the average, 50 percent of the coating particles fed to the HVOF gun are deposited on the substrate. This relatively low deposition efficiency of the HVOF spraying systems can be the result of having many particles among the particulate flow with velocities smaller than the critical velocity. The interaction of oblique shock and expansion wave with solid particles is shown in FIG. 3.
Another drawback of the current HVOF nozzle design of FIG. 1 relates to the degree of oxidation of in-flight particles. While high particle kinetic energy upon impact leads to formation of a dense, well-adhered coating, in contrast, low temperature prevents the in-flight particles from extensive oxidation resulting in coatings with lower oxygen content. Any thermal spray process in ambient atmosphere is accompanied by air entrainment which results in in-flight metal particle oxidation. It is recognized that minimizing oxidation during the coating operation results in improvement of overall coating performance. Vacuum plasma spraying (VPS) allows one to reduce or eliminate oxygen in the spraying region and provides oxide-free coatings, but this process is expensive, time consuming and has restriction on the size of coated parts by the size of the vacuum chamber. Compared to other spraying processes, oxidation rate during the HVOF spraying is one of the lowest and under certain conditions, it is comparable with that of the VPS coatings. In order to use the HVOF process as a technological alternative to the cost intensive VPS process, air entrainment should be minimized.
A further drawback of the present HVOF deposition gun relates to the types of materials that can be deposited. Due to the low flame temperatures, HVOF cannot be used for ceramic coatings. It is primarily used in spraying metals or carbides with metallic binders.
Although the HVOF process has shown to be a technological alternative to the many conventional thermal spray processes, it would be very advantageous to provide a deposition nozzle that provides improved performance in the areas of deposition efficiency, coating oxidation, and flexibility to allow coating of ceramic powders.